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Date:   Tue, 19 Jan 2021 19:34:49 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-unionfs@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, "Serge E . Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH 2/2] security.capability: fix conversions on getxattr

Miklos Szeredi <mszeredi@...hat.com> writes:

> If a capability is stored on disk in v2 format cap_inode_getsecurity() will
> currently return in v2 format unconditionally.
>
> This is wrong: v2 cap should be equivalent to a v3 cap with zero rootid,
> and so the same conversions performed on it.
>
> If the rootid cannot be mapped v3 is returned unconverted.  Fix this so
> that both v2 and v3 return -EOVERFLOW if the rootid (or the owner of the fs
> user namespace in case of v2) cannot be mapped in the current user
> namespace.

This looks like a good cleanup.

I do wonder how well this works with stacking.  In particular
ovl_xattr_set appears to call vfs_getxattr without overriding the creds.
What the purpose of that is I haven't quite figured out.  It looks like
it is just a probe to see if an xattr is present so maybe it is ok.

Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

>
> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> ---
>  security/commoncap.c | 67 ++++++++++++++++++++++++++++----------------
>  1 file changed, 43 insertions(+), 24 deletions(-)
>
> diff --git a/security/commoncap.c b/security/commoncap.c
> index bacc1111d871..c9d99f8f4c82 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -371,10 +371,11 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
>  {
>  	int size, ret;
>  	kuid_t kroot;
> +	__le32 nsmagic, magic;
>  	uid_t root, mappedroot;
>  	char *tmpbuf = NULL;
>  	struct vfs_cap_data *cap;
> -	struct vfs_ns_cap_data *nscap;
> +	struct vfs_ns_cap_data *nscap = NULL;
>  	struct dentry *dentry;
>  	struct user_namespace *fs_ns;
>  
> @@ -396,46 +397,61 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
>  	fs_ns = inode->i_sb->s_user_ns;
>  	cap = (struct vfs_cap_data *) tmpbuf;
>  	if (is_v2header((size_t) ret, cap)) {
> -		/* If this is sizeof(vfs_cap_data) then we're ok with the
> -		 * on-disk value, so return that.  */
> -		if (alloc)
> -			*buffer = tmpbuf;
> -		else
> -			kfree(tmpbuf);
> -		return ret;
> -	} else if (!is_v3header((size_t) ret, cap)) {
> -		kfree(tmpbuf);
> -		return -EINVAL;
> +		root = 0;
> +	} else if (is_v3header((size_t) ret, cap)) {
> +		nscap = (struct vfs_ns_cap_data *) tmpbuf;
> +		root = le32_to_cpu(nscap->rootid);
> +	} else {
> +		size = -EINVAL;
> +		goto out_free;
>  	}
>  
> -	nscap = (struct vfs_ns_cap_data *) tmpbuf;
> -	root = le32_to_cpu(nscap->rootid);
>  	kroot = make_kuid(fs_ns, root);
>  
>  	/* If the root kuid maps to a valid uid in current ns, then return
>  	 * this as a nscap. */
>  	mappedroot = from_kuid(current_user_ns(), kroot);
>  	if (mappedroot != (uid_t)-1 && mappedroot != (uid_t)0) {
> +		size = sizeof(struct vfs_ns_cap_data);
>  		if (alloc) {
> -			*buffer = tmpbuf;
> +			if (!nscap) {
> +				/* v2 -> v3 conversion */
> +				nscap = kzalloc(size, GFP_ATOMIC);
> +				if (!nscap) {
> +					size = -ENOMEM;
> +					goto out_free;
> +				}
> +				nsmagic = VFS_CAP_REVISION_3;
> +				magic = le32_to_cpu(cap->magic_etc);
> +				if (magic & VFS_CAP_FLAGS_EFFECTIVE)
> +					nsmagic |= VFS_CAP_FLAGS_EFFECTIVE;
> +				memcpy(&nscap->data, &cap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
> +				nscap->magic_etc = cpu_to_le32(nsmagic);
> +			} else {
> +				/* use allocated v3 buffer */
> +				tmpbuf = NULL;
> +			}
>  			nscap->rootid = cpu_to_le32(mappedroot);
> -		} else
> -			kfree(tmpbuf);
> -		return size;
> +			*buffer = nscap;
> +		}
> +		goto out_free;
>  	}
>  
>  	if (!rootid_owns_currentns(kroot)) {
> -		kfree(tmpbuf);
> -		return -EOPNOTSUPP;
> +		size = -EOVERFLOW;
> +		goto out_free;
>  	}
>  
>  	/* This comes from a parent namespace.  Return as a v2 capability */
>  	size = sizeof(struct vfs_cap_data);
>  	if (alloc) {
> -		*buffer = kmalloc(size, GFP_ATOMIC);
> -		if (*buffer) {
> -			struct vfs_cap_data *cap = *buffer;
> -			__le32 nsmagic, magic;
> +		if (nscap) {
> +			/* v3 -> v2 conversion */
> +			cap = kzalloc(size, GFP_ATOMIC);
> +			if (!cap) {
> +				size = -ENOMEM;
> +				goto out_free;
> +			}
>  			magic = VFS_CAP_REVISION_2;
>  			nsmagic = le32_to_cpu(nscap->magic_etc);
>  			if (nsmagic & VFS_CAP_FLAGS_EFFECTIVE)
> @@ -443,9 +459,12 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
>  			memcpy(&cap->data, &nscap->data, sizeof(__le32) * 2 * VFS_CAP_U32);
>  			cap->magic_etc = cpu_to_le32(magic);
>  		} else {
> -			size = -ENOMEM;
> +			/* use unconverted v2 */
> +			tmpbuf = NULL;
>  		}
> +		*buffer = cap;
>  	}
> +out_free:
>  	kfree(tmpbuf);
>  	return size;
>  }

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